Firing circuit for blasting caps

ABSTRACT

The present system for selectively firing explosive charges utilizes as firing a signal an alternating current having a frequency of the order of 10 kHz and a voltage of at least 60 volts, which is transmitted by cable to the explosive site. A discriminating circuit adjacent each firing squib includes a step-down transformer with saturable core which effectively blocks spurious 60 cycle alternating current voltages. Series and shunt capacitances further insure against spurious signals and tune the circuit to enhance response to the firing signal. Convenient testing and monitoring circuits are described.

United States Patent 1 Vlahos 1 Oct. 2, 1973 I FIRING CIRCUIT FOR BLASTING CAPS Primary Examiner-Benjamin A. Borchelt I Assistant ExaminerThomas H. Webb [75] Inventor. l'etro Vlahos, Fur/:ana, Calif. Anomey chaflton M. Lewis [73] Assignee: The Association of Motion Picture and Television Producers Inc., [57] ABSTRACT Honywod* Cuhf' The present system for selectively firing explosive 22] Filed: Man 29 1972 charges utilizes as firing a signal an alternating current having a frequency of the order of kHz and a voltage, I l PP 9,263 of at least 60 volts, which is transmitted by cable to the explosive site. A discriminating circuit adjacent each [52] 102/701 R, 317/80, 102/28 firing squib includes a step-down transformer with satu- I5H Int CL. F42: 11/00 F42c 19], F42c /40 rable core which effectively blocks spurious cycle 58 Field of Search 102/702 70.2 GA; alternating Current voltages- Series and Shunt capaci- 320'H; 317/ tances further insure against spurious signals and tune the circuit to enhance response to the firing signal. [56] Reierences Cited Convenient testing and monitoring circuits are de- 'UNITED STATES PATENTS Scnbed' 3,670,653 6/1972 Lunt 102 702 R 9 Claims, 1 Drawing Figure 3,682,098 8/1972 Spies 102/702 R FIRING CIRCUIT FOR BLASTING CAPS This invention concerns the remote firing of explosives under electrical control.

The primary purpose of the invention is to produce reliable firing of a detonating device under electrical control, while preventing unintentional firing due to accidental voltage application.

The invention is intended particularly but not exclusively for such applications as producing controlled explosions during the filming of motion pictures. Complete elimination of spurious firing signals is particularly difficult on a motion picture set because of the wide variety of electrical apparatus that must be employed in complex and ever-changing configurations. The result is typically a maze of electrical cables interconnecting portable devices of many kinds. A multitude of 120/240 volt D.C. power cables for are lights intermesh with 120/240 volt A.C. power cables, telephone lines and blasting wires. That changing grid is subjected to the continuous traffic of 50 or more persons milling about with camera dollies and other vehicles, criss-crossing over the cables and wires, often in mud and water. At the same time, dry blowing winds on many locations promote accumulations of static electricity which can discharge to any conductive material in the vicinity. Radio communications equipment and powerful radar beams from nearby military installations can be accidentally picked up by circuits of many types.

The problem is to generate and transmit a firing signal through such an environment and sometimes over a total distance of half a mile or more to fire an explosive igniter with absolute reliability and with positive exclusion of spurious response. That means, in practice, that the firing circuit must not only be immune to currents induced electromagnetically or conducted by leakage through damp ground from neighboring power lines. It must even be immune to direct electrical contact with bare wires in any of the cable types that have been mentioned. And the firing circuit must be immune to static electricity discharged directly to it. The difficulty of maintaining immunity of all those types will be realized from the fact that a typical explosive igniter will fire in response to from 0.4 to 0.7 volt and can be fired by a single cell penlight battery.

The present invention solves that problem by generating on command at the control station an alternating current firing signal having a frequency of the order of kHz and a voltage of at least about 60 volts. That signal is transmitted by cable to the site of the explosive charge, where it enters a discriminating unit. That unit blocks any spurious signals received from the cable, while transforming the firing signal down to suitable voltage for firing a conventional squib. The discriminating unit typically includes a step-down transformer, a series capacitance for blocking all direct current voltages and a high frequency shunting circuit for bypassing radio frequencies and providing resonance at the signal frequency for enhancing the circuit response to the firing signal. The transformer core is so designed that it becomes nearly saturated in response to the firing signal, but normally produces an output signal of about 1.5 volt. The transformer secondary is preferably enclosed by a grounded shield.

With that relatively simple and economical control system, from one to about four squibs can be fired simultancously by each discriminator unit, and a single control station can readily distribute firing signals as required to a large number of discriminator units at distances up to half a mile or more. Spurious D.C. voltages are blocked by the series capacitance and by the stepdown transformer. AC. power frequencies are strongly attenuated by the series capacitance and by saturation of the transformer core, so that even direct connection to the firing line does not produce a firing voltage in the transformer secondary. Radio frequencies are shunted around the transformer, and may be further attenuated by a series choke coil, if desired. Any remaining R.F. voltage is rendered completely ineffective by the great voltage reduction in the transformer. Static discharges are similarly attenuated, and, despite their high voltage, do not contain sufficient energy to produce an effective firing current.

A particular advantage of the present system is that the distinctive firing signal which is transmitted the entire length of the firing line is not merely an enabling signal, as in some prior art systems, but carries the entire firing power. Hence pickup by the firing line of stray frequencies close to the value of the firing signal are inherently inadequate to fire a squib. That is because the signal frequency is in the high audio range where any accidental pickup is wholly inadequate either in voltage level or power level. Yet the present firing signal can transport the needed power over conveniently small wires due to its relatively high voltage level of 60 to 240 volts. Moreover, since the firing signal is A.C., the firing line is unpolarized and cannot be connected with wrong polarity.

A full understanding of the invention, and of its further objects and advantages, will be clear from the following description of an illustrative system for carrying it out. That description is to be read with reference to the accompanying drawing, which is a schematic diagram representing a preferred embodiment of the invention. The particulars of that drawing and of the accompanying description are intended only as illustration of the invention.

The illustrative preferred embodiment of the invention shown schematically in FIG. 1 comprises several separate units which are connected by flexible cables to form a complete system. That arrangement, however, may be varied widely to suit particular requirements. The main firing box or signal generator is indicated at 10. The enabling unit 12 contains the enabling switch S1 and is connected to firing box 12 by the short 3-wire flexible cable 13 and the connector 14. The selector unit 16 contains several selector switches S2 by which the firing signal may be directed to a desired one or group of explosive charges to be fired. Selector 16 is connected to firing box 10 via the short flexible cable 17 and the connector 18. The firing box, enabling unit and selector unit are all located relatively close to each other at the command station, which is typically from to 1,000 yards from each of the firing stations or explosive sites.

The output from each selector switch S2 is connected via a cable 22 to a signal discriminator 20 at a firing station. That discriminator eliminates false or spurious voltages which may be received from line 22, and transforms the firing signal to a form suitable for delivery via the line 23 to a conventional squib or igniter 24. Squib 24 is positioned immediately adjacent and in firing relation to the explosive charge 25 to be fired.

Firing box 10 includes a suitable source of electrical power, shown as the battery B, typically delivering about 12 volts with the polarity indicated. The positive power line from battery B passes through the resistance R1 to contact A of connector 14. When connector 14 is in working position, as shown, terminal A is connected continuously via cable 13 and enabling unit 12 to terminal B, arming the system. Terminal A is then connectible at will to terminal C upon manual closure of enabling switch S1. The winding of relay Ry is connected between terminal B and the negative power line 32, with the diode D1 in series between that power line and the winding. Ry controls the relay switches S3 and S4, which are normally closed, as shown. The signal generator 30 has its positive input terminal 1 connected to terminal C and its negative terminal 2 connected via the junction 34 and the diode D2 to negative power line 32. The large capacitor C1, typically of the order of l00,000 MFD, is connected directly between junction 34 and terminal B. Capacitor C1 is shunted by the voltmeter V, and also by the resistance R2 and relay switch S3, connected in series.

The output terminals 3 and 4 of signal generator 30 are connected directly to the connector terminals D and E for supplying the firing signal to cable 17 via conector 18. The output line is shunted via the relay switch S4, and is also shunted via the indicator lamp D81 and the current limiting resistance R3.

A useful test facility is provided by the two additional connector terminals F and G, which are connected, respectively, to negative power line 32 and via the resistance R4 to the normally closed terminal of the manual switch S5. The normally open terminal of that switch is connected via the resistance R5 to the positive terminal of battery B. The common switch terminal is connected via the indicator lamp D82 to negative power line 32.

In accordance with an important aspect of the present invention, signal generator 30 responds to power application by generating at terminals 3 and 4 an alternating current firing signal having a frequency of the order of kHz. That frequency is preferably quite stable, but its precise value is not critical so long as other parts of the system conform properly to the value selected. When the system is to be use in connection with production of motion pictures, the frequency selected is preferably spaced above the highest frequencies to which the audio recording equipment might respond, that is, typically above about 7,000 Hz. On the other hand, the selected frequency should be well below the lowest frequencies that might be picked up accidentally from radio signals, which start at about 100 kHz. Values between about 7,000 and about 10,000 Hz are generally preferred, such as 8,500 Hz, for example. The voltage of the firing signal is made at least one, and preferably two, orders of magnitude higher than the firing voltage of a squib, but not appreciably higher than conventional electrical power circuit voltages. in practice, such a range is between about 60 and about 240 volts, about 130 volts being a preferred value. For clarity of description, a firing signal of 8,500 Hz and 130 volts will be assumed in general in the present disclosure.

Signal generator 30 typically comprises an inverter circuit, including a solid state electronic oscillator for producing the required frequency, and a transformer for stepping up the resulting alternating current voltage to the desired level. Circuitry is preferably provided for stabilizing the frequency and voltage of the output signal. The output circuit is preferably tuned to the signal frequency to suppress transmission of other frequencies. Such inverter circuits are well known and do not require detailed description here.

Selection unit 16 receives the firing signal from firing box 10 via the cable 17 and permits its convenient distribution to any of the output circuits, represented by the respective pairs of binding posts 38, to which output cables 22a, 22b, 22c, etc., are connected. Cables 17 and 22 typically require only two wires each. One binding post of each pair is connected to one line of input cable 17, while the other post is connected to the other input line via the corresponding selection switch S2. Thus any selected output cable or cables 22 can be connected in parallel to input cable 17 by closure of the corresponding switches S2. Switches S2, and also switch S1 in enabling unit 12, are of the known pushbutton type which remains closed only while manually depressed. Any desired number of output circuits 22 may be provided, and the multiple manual selection switches S2 of the figure may be replaced, if desired, by a stepping switch with manual or other control in the manner well known in the art. 1

Each discriminator circuit 20, as typically shown, comprises the transformer T with its primary winding 42 coupled to the input terminals 43 and 44 via the series capacitance C2 and the choke coil Ll. Primary winding 42 is shunted by the capacitance C3. The input terminals 43 and 44 are shunted by the series connected resistances R 5 and R7, which are preferably equal and have their junction grounded via the line 46. The ground connection itself, indicated schematically at 47, preferably comprises an actual local connection to the earth, such as a metal rod driven directly into the ground. The secondary winding 48 of transformer T, typically comprising only a few turns, is preferably surrounded by a grounded electrostatic shield 49. This winding is directly connected to the output terminals 50. The very low resistance of the secondary winding acts as a permanent shunt across the squib for DC or 60 cycle A.C. voltages.

Squib 24 may be of any type that responds to the selected firing voltage. it comprises an igniter charge and an electrical resistance wire 26 of suitable form to set off the igniter charge in response to an applied voltage of predetermined magnitude, typically requiring between 0.4 and 0.7 volts. It is connected to terminals 50 via the cable 23, which is preferably made of twisted wire to reduce induction of voltages, and is preferably as short as is feasible.

In operation of firing box 10, when connector 14 is disconnected the battery circuit is open at terminal A and at switch S5 and the system is therefore completely idled. Relay Ry is deenergized, as shown, with relay switch S3 shunting Cl and therefore insuring its full discharge, and with relay switch S4 shorting the output terminals D and E and any external circuit connected thereto.

As soon as the operator plugs in connector 14, to arm the system, Ry is energized, opening S3 and S4. Current flows to C1 via the current limiting resistance R1, which is typically only a fraction of an ohm, charging C1 to the full voltage of battery B, which may be read on voltmeter V. However, power supply to signal generator 30 remains open at S1. Upon manual closure of S1 the battery voltage is applied to the signal generator both from the battery and from the charge stored in C1, insuring an effectively low impedance supply of power sufficient to fire several squibs. The resulting firing signal from generator 30 appears at terminals D and E and is indicated by glowing of the neon indicator lamp DSl. However, that Signal cannot reach squib 24 until at least one of the selector switches S2 is closed. Since the operator must hold S1 closed with one hand and close the selected switch S2 with the other hand, accidental closure of both switches is effectively prevented. Upon release of enabling switch S1, power is immediately deleted from signal generator 30 and any charge that has been withdrawn from C1 is restored from the battery. After completion of a series of explosions, the operator preferably idles the system by removing connector 14. Ry is thereby deenergized, discharging Cl via closed S3 and R2. That discharge is readily verified on voltmeter V. Diodes D1 and D2 prevent continued energization of Ry by discharge current from C1.

Before connecting a discriminating circuit 20 to a squib and to the relatively long cable 22, it is useful to test it in combination with firing box 10. For that purpose, output terminals 50 of the discriminating circuit are connected to test terminals F and G of the firing box, cable 22 being replaced by any suitable connections between input terminals 43 and 44 of the discriminating circuit and terminals 38 of 16.. Proper operation of the system is then indicated by illumination of indicator lamp Ds2 upon simultaneous closure of S1 and S2. if that lamp should not respond, its condition may be checked by operating switch S5.

Turning now to discriminator 20, primary and secondary windings 42 and 48 of transformer T are preferably wound on the transformer core with physical separation, insulation and a metallic shield between them to reduce the possibility of an accidental short between them from a static discharge of up to 20,000 volts. The ferromagnetic transformer core is designed, in accordance with known electromagnetic laws, to become nearly saturated in response to the voltage and frequency of the normal firing signal, and to transform that signal from its input voltage of about 130 volts to an output value of about twice the normal firing voltage for the squib, or typically 1.5 volts. That voltage reduction of roughly 100:1 is accompanied by a corresponding current increase, so that the firing current drawn by the squib, typically about 1.5 amperes, requires a relatively low current of the order of only about ma in the transformer primary and in the long connecting cable 22. Hence the voltage drop in that line is relatively small. In actual practice, a firing signal can be transmitted successfully up to half a mile through wire as small as No. l6, for example.

Series choke L1 is typically selected with regard to the effective capacitance of cable 22 to improve the impedance match. It offers little impedance to the firing signal, but appreciably attenuates radio frequencles.

Capacitance C3, which shunts the transformer primary winding, is selected with respect to the effective reactance of the transformer to produce resonance of the circuit at the frequency of the firing signal. All other frequencies are then automatically attenuated relative to the firing signal. C3 also provides a low impedance shunt for bypassing radio frequencies around the transformer. Any radio frequency energy is blocked by choke L1, bypassed by C3, or is so reduced in voltage by the transformer as to be ineffective. In practice even if cable 22 is replaced by a tuned antenna adjacent to a 50,000 watt radio transmitter, the energy reaching the squib is found to be well below that required to fire it.

Series capacitance C2 is seen as an open circuit by any spurious direct current voltages, and as a high impedance by any 60 cycle voltage. Even if the capacitor should fail, the transformer does not pass direct current, and the transformer core becomes saturated by a 60 cycle voltage of only about 1 volt, so that even 220 volts applied accidentally from a power line can produce only about l/l00 volt in the transformer secondary, far below the effective firing voltage.

Resistors R6 and R7, through which the lead wires entering discriminator 20 are shunted and grounded, are typically of the order of 24,000 ohms each. These resistors are effective in bleeding to ground any static voltages that could otherwise build upon the long cable 22. They also permit a simple test for continuity of the circuit back to firing box 10, as by applying a direct current test signal at connector 18. Such a test can safely be made even with a firing squib connected and in firing position.

A particular advantage of utilizing a step-down transformer as described above is its action in protecting the igniter from spurious voltages induced in connecting cable 23. Since the transformer secondary winding typically comprises only one or two turns, its resistance to DC. voltages can readily be made a very small fraction of the igniter resistance, which is typically about 1 ohm. If that fraction is made less than about one tenth, the shunting action of the secondary winding provides appreciable protection of the igniter from stray direct current or low frequency voltages, and that protection increases as the described fraction becomes smaller.

When the transformer core is designed to become saturated in the manner already described, the effective impedance of the secondary winding to all A.C. voltages that produce such saturation is also a small fraction of the igniter impedance. For the normal power line frequency of 60 Hz, saturation typically occurs at secondary voltages that exceed about 1/10 volt, and preferably at voltages that exceed only about 1] volt, values well below the igniter firing voltage. The igniter is thus effectively protected against pickup from power lines by the cable connecting the igniter to the transformer.

It is noted, particularly, that the present system derives the fullelectrical power for firing the squib or squibs directly from the high frequency firing signal. It is conceivable that stray voltages close to the signal frequency of about 8,500 Hz might be induced in the long cables 22, for example from audio equipment in the vicinity. However, either the voltage or the actual power transferred to the signal circuit by such induction would always be small, and completely inadequate to produce in the transformer secondary sufficient power to fire a squib.

It will be understood that many modifications can be made in the particulars of the described system, which is intended only as illustration. For example, electrical power for inverter 30 can be derived from a conventional direct or alternating current power line, rather than from a battery such as B. In that case, Cl and relay switch S3 can be omitted, making D1 and D2 unnecessary, and inverter 30 is modified appropriately.

I claim 1. A firing circuit for igniting an explosive charge at a firing station, comprising circuit means at a control station remote from the firing station for selectively generating an alternating current firing signal having a signal voltage between about 60 and about 240 volts and a signal frequency of the order of kHz,

conductive circuit means for selectively transmitting the firing signal from the control station to the firing station,

an electrical igniter at the firing station in firing relation to the explosive charge and responsive to a firing voltage of the order of H100 of said signal voltage,

a capacitance,

a step-down transformer adjacent the firing station having a primary winding coupled to said conductive circuit means via a series connection of said capacitance to receive the firing signal, and having a secondary winding electrically coupled to the igniter and electromagnetically coupled to the primary winding to supply to the igniter sufficient power to fire the same at a voltage approximately twice said firing voltage, said power being derived from said transformer in response to the firing signal.

2. A firing circuit according to claim 1, and wherein said primary and secondary transformer windings are electromagnetically coupled by ferromagnetic core means of such characteristics as to be fully saturated in response to application to the primary winding of a 60 Hz alternating voltage of the order of one volt.

3. A firing circuit according to claim 1, and including a resistive shunt connected across said conductive circuit means at the firing station,

and means for locally grounding the midpoint of the resistive shunt.

4. A firing circuit according to claim 1, and including reactance means connected in series with said conductive circuit means adjacent the firing station for attenuating spurious radio frequency signals.

5. A firing circuit according to claim 1, and in which said signal generating circuit means have such characteristics that said firing signal has a voltage of approximately 130 volts and a frequency of approximately 8,500 Hz.

6. A firing circuit according to claim 1, and in which said secondary winding is directly connected to the igniter and forms a shunt circuit in parallel with the igniter having a resistance less than about one tenth the resistance of the igniter.

7. A firing circuit according to claim 2, and in which said secondary winding is directly connected to the igniter and forms a shunt circuit in parallel with the igniter, the shunt impedance to A.C. voltages that are of power frequency and that exceed about l/lO volt being less than about one tenth the impedance of the igniter.

8. A firing circuit for igniting an explosive charge at a firing station, comprising circuit means at a control station remote from the firing station for selectively generating an alternating current firing signal having a signal voltage between about 60 and about 240 volts and a signal frequency of the order of 10 kHz,

conductive circuit means for selectively transmitting the firing signal from the control station to the firing station,

an electrical igniter at the firing station in firing relation to the explosive charge and responsive to a firing voltage of the order of H of said signal voltage,

a step-down transformer adjacent the firing station having a primary winding coupled to said conductive circuit means and a secondary winding coupled to the igniter,

and a capacitance conneced in shunt to the transformer primary winding and forming therewith a circuit resonant at said signal frequency.

9. A firing circuit according to claim 8, and including a second capacitance, said transformer primary winding being coupled to said conductive circuit means via a series connection of said second capacitance. 

2. A firing circuit according to claim 1, and wherein said primary and secondary transformer windings are electromagnetically coupled By ferromagnetic core means of such characteristics as to be fully saturated in response to application to the primary winding of a 60 Hz alternating voltage of the order of one volt.
 3. A firing circuit according to claim 1, and including a resistive shunt connected across said conductive circuit means at the firing station, and means for locally grounding the midpoint of the resistive shunt.
 4. A firing circuit according to claim 1, and including reactance means connected in series with said conductive circuit means adjacent the firing station for attenuating spurious radio frequency signals.
 5. A firing circuit according to claim 1, and in which said signal generating circuit means have such characteristics that said firing signal has a voltage of approximately 130 volts and a frequency of approximately 8,500 Hz.
 6. A firing circuit according to claim 1, and in which said secondary winding is directly connected to the igniter and forms a shunt circuit in parallel with the igniter having a resistance less than about one tenth the resistance of the igniter.
 7. A firing circuit according to claim 2, and in which said secondary winding is directly connected to the igniter and forms a shunt circuit in parallel with the igniter, the shunt impedance to A.C. voltages that are of power frequency and that exceed about 1/10 volt being less than about one tenth the impedance of the igniter.
 8. A firing circuit for igniting an explosive charge at a firing station, comprising circuit means at a control station remote from the firing station for selectively generating an alternating current firing signal having a signal voltage between about 60 and about 240 volts and a signal frequency of the order of 10 kHz, conductive circuit means for selectively transmitting the firing signal from the control station to the firing station, an electrical igniter at the firing station in firing relation to the explosive charge and responsive to a firing voltage of the order of 1/100 of said signal voltage, a step-down transformer adjacent the firing station having a primary winding coupled to said conductive circuit means and a secondary winding coupled to the igniter, and a capacitance conneced in shunt to the transformer primary winding and forming therewith a circuit resonant at said signal frequency.
 9. A firing circuit according to claim 8, and including a second capacitance, said transformer primary winding being coupled to said conductive circuit means via a series connection of said second capacitance. 